Fix build before 4.12

Author: OlivierNicole

compiler/lib/ocaml_compiler.ml

@@ -42,7 +42,7 @@ let rec constant_of_const ~target c : Code.constant =
       Int
         (match target with
         | `JavaScript -> Int32.of_int_warning_on_overflow i
-        | `Wasm -> Int31.of_int_warning_on_overflow i)
+        | `Wasm -> Int31.(of_int_warning_on_overflow i |> to_int32))
   | Const_block (tag, l) ->
       let l = Array.of_list (List.map l ~f:(fun c -> constant_of_const ~target c)) in
       Tuple (tag, l, Unknown)